1. Field of the Invention
The present invention relates to a heat-sensitive recording material. More particularly, the present invention relates to a heat-sensitive recording material having a superior storage durability, a high water-resistance of the heat-sensitive color-forming layer thereof and color-developed images thereon, and a superior resistance of the color-developed images to color-fading when the color-developed images on the heat-sensitive color-forming layer are brought into contact with a thermo-plastic resin film containing a plasticizer and with an oily substance, for example, an edible meat, oil or fat, respectively.
2 Description of the Related Arts
Conventional heat-sensitive recording materials produced by coating a surface of a substrate sheet with a coating material prepared by dispersing a mixture of a dye precursor consisting of a color-forming lactone compound, for example, crystal violet lactone, and a color-developing agent consisting of an organic acid or phenol compound, for example, bisphenol A, in a solution of a binder, for example, polyvinylalcohol, are disclosed in various prior art publications, for example, Japanese Examined Patent Publication (Kokoku) No. 45-14,039 and U.S. Pat. No. 3,539,375. When this type of heat-sensitive recording materials is heated, the color-forming lactone compound chemically reacts with the color-developing agent, for example, the organic acid or phenol compound, to form colored images. These heat-sensitive recording materials must meet the following requirements:
(1) The colored images formed on the recording material must be clear and have a good color depth.
(2) The appearance of the recording materials must be close to that of conventional white paper sheets.
(3) The recording materials must be able to be stably stored over a long period.
(4) The heat-sensitivity of the recording materials must be satisfactory.
(5) The recording materials must not stick or adhere to each other or to a thermal head of a printing machine in which the recording materials are thermally printed.
(6) The recording materials must not cause the thermal head to be stained by grounds derived therefrom, and thus must have a good applicability to the thermal head.
The conventional heat-sensitive color-forming layers of the recording materials, however, are sensitive and not stable to light, water, heat, plasticizers and oily substances, and thus are deteriorated during storage thereof. Namely, where the conventional heat-sensitive recording materials are subjected to a thermal recording procedure, the color-forming layer can form clear colored images thereon, whereas where a thermoplastic resin film, for example, a soft polyvinyl chloride resin film, containing a certain amount of a plasticizer or an oily substance, for example, edible meat, oil or fat, is brought into close contact with a heat sensitive color-forming layer, the colored images formed on the color-forming layer are faded. Also when the color-forming layer is in contact with water over a long time, the colored images thereon are significantly faded.
The above-mentioned color-fading phenomenon is conspicuous in the color-developing type heat-sensitive recording materials in which a dye precursor comprising, as a main component, a colorless or light colored color-forming lactone compound, is used as a color-forming component.
It is assumed that the color-fading phenomenon of the colored images is not derived from the specific chemical structures of the color-forming lactone compound but is due to the fact that, when brought into contact with the thermoplastic resin films containing a plasticizer or with the oily substances, the color-developing agent, for example, an organic acid or phenol compound, is dissolved in the plasticizer in the thermoplastic resin film or in the oily substances. Therefore, there is a strong demand for a method of preventing of the color-fading of heat-sensitive recording materials.
To prevent color-fading, an attempt has been made to arrange a protective coating layer on the heat-sensitive color-forming layer in the heat-sensitive recording material. The protective coating layer comprises a water-soluble polymeric material, for example, polyvinylalcohol or casein, which can form a barrier against the plasticizers and oily substances, and a cross-linking agent. Nevertheless, even when the above-mentioned type of conventional protective coating layer is formed on the heat-sensitive color-forming layer, when brought into close contact with the plasticizer-containing plastic films or the oily substances, the colored images formed on the color-forming layer disappear within a time of from 0.5 to 5 hours, and thus the protection offered by the conventional protecting coating layer is unsatisfactory.
Japanese Unexamined Patent Publication (Kokai) No. 58-188687 and U.S. Pat. No. 4,485,139 disclose a heat-sensitive recording material in which a protective coating layer containing a mixture of a polyvinylalcohol resin and boric acid is arranged on a heat-sensitive color-forming layer, but this type of recording material is disadvantageous in that, in the formation of the protective coating layer, a vigorous gelation reaction occurs in the coating liquid containing the polyvinylalcohol resin and boric acid, and thus the coating liquid exhibits a very poor stability and low durability (pot life). Japanese Unexamined Patent Publication (Kokai) No. 59-106,995 and U.S. Pat. No. 4,513,301 disclose a heat-sensitive recording material having a protective coating layer comprising, as a main component, at least one polymeric resin selected from acetoacetylated polyvinylalcohol resins and acrylonitrile-copolymerized polyvinylalcohol resins and a cross-linking agent, and arranged on a heat-sensitive color-forming layer. This type of the protective coating layer is disadvantageous in that the reaction between the polymeric resin and the cross-linking agent in a coating liquid for the protective coating layer is vigorous and causes a very rapid increase of the viscosity of the coating liquid. Therefore, it is impossible to smoothly and stably carry out the coating operation for the protective coating layer, and the coating liquid exhibits a low durability (pot life) due to the gelation thereof.
Japanese Unexamined Patent Publication (Kokai) No. 60-260,382 discloses a coating liquid to be applied to a heat-sensitive color-forming layer, which coating comprises a polyvinylalcohol resin aqueous solution to which a dehydration reaction product of 2 molar parts of glycerol with one molar part of boric acid is added. This coating liquid is disadvantageous in that it has a low storage durability (pot life), and thus is not suitable for practical use.
Japanese Unexamined Patent Publication (Kokai) No. 1-120,385, to the same inventors as in the present invention, discloses a protective coating layer coated on the heat-sensitive color-forming layer and comprising a polyvinylalcohol resin, a polyvinyl-alcohol derivative resin having acetoacetic ester groups attached to polyvinylalcohol polymeric molecules and a water soluble cross-linking agent reactive with the above-mentioned resins. This type of protective coating layer is not always satisfactory, and thus requires improvement.
Under the above circumstances, there is a strong demand that the above-mentioned disadvantages of the conventional heat-sensitive recording materials be eliminated.